Lithography tool image quality evaluating and correcting

ABSTRACT

Electron beam lithography tool image quality evaluating and correcting including a test pattern with a repeated test pattern cell, an evaluation method and correction program product are disclosed. The test pattern cell includes a set of at least three elongated spaces with each elongated space having a different width than other elongated spaces in the set such that evaluation of a number of space widths in terms of tool image quality and calibration can be completed. The evaluation method implements the test pattern cell in a test pattern in at least thirteen sub-field test positions across an exposure field, which provides improved focus and astigmatism corrections for the lithography tool. The program product implements the use of corrections from the at least thirteen sub-field test positions to provide improved corrections for any selected sub-field position.

This application is a division of application Ser. No. 10/604,051 filedJun. 24, 2003 now U.S. Pat. No. 6,931,337, currently pending.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to lithography tools, and moreparticularly, to electron beam lithography tool image quality evaluatingand correcting.

2. Related Art

Image quality optimization on an electron beam lithography tool involvesthe adjustment of many electron optical parameters to correct forresolution limiting effects. Illustrative parameters include filamentpower, anode position, lens excitations, shape aperture rotation, shapedeflection calibration, focus coil excitation, stigmator correctorexcitations and beam exposure time. Contributing resolution detractorsmust be determined from the resultant exposed images. Resolutionlimiting effects may include, for example, illumination non-uniformity,shape aperture image rotation errors, shape aperture image size errors,shape deflection miscalibration, spot defocus, spot astigmatism, focusvariations and dose variations.

Conventional methodology to provide image quality optimization includesimplementation of a test pattern, having a specialized test patterncell, within an exposure field to highlight potential problems, and thendetermination of appropriate image quality corrections of thelithography tool to address the identified problems. The magnitude ofsome resolution detractors, such as spot shape errors, spot illuminationnon-uniformities and beam dose errors are typically independent oflocation in the exposure field. Accordingly, correction requiresevaluation of exposed images for consistent errors across the field, andthen applying uniform corrections. In contrast, other resolutionlimiting factors such as spot defocus and astigmatism lead to imagequality variations across the exposure field. Corrections for theseimage quality problems have to be a function of position in the exposurefield. The exposure field may be made up of thousands of sub-fields(e.g., a 2.16 mm×2.16 mm exposure field can be broken up into 90×90array of 24 micron sub-fields). Typically, exposure of the test patternincludes exposure using an identical test pattern cell at nine (9)sub-field positions of an exposure field. Assuming a square exposurefield, the test pattern cell may be used at the four corner sub-fieldpositions, vertical and horizontal intermediate sub-field positionsbetween the four corner sub-field positions, and a center sub-fieldposition. Focus and astigmatism corrections for sub-field positionsbetween these nine sub-field positions are provided by linearinterpolation of correction data for the two nearest sub-fieldpositions, which introduces inaccuracy in the corrections. For example,for a sub-field position half-way between a corner sub-field positionand the center sub-field position, an averaging of the corrections forthose respective sub-field positions would be used.

Additional problems relative to image quality optimization result fromconventional test pattern cells. In particular, each test pattern cellis typically provided with one or two unique space widths, whichnecessitates numerous tests to address problems with a range of spacewidths. For example, an illustrative test pattern cell 10 is shown inFIG. 1. Test pattern cell 10 includes a plurality of elongated spaces 11extending from a border 14 , another plurality of elongated spaces 12extending from border 14 , an array of varied sized square spaces 16 ,and an X pattern cell 18 and a cross pattern cell 20 of the same widthas elongated spaces 12. In terms of elongated spaces, only two differentwidths are provided. Use of such a limited number of unique elongatedspace widths in a test pattern cell presents problems because the choiceof appropriate corrections to improve image quality may depend on theimage fidelity over a range of space widths. For example, exposure dosecalibration errors can result in space width errors that increase ordecrease as a function of nominal feature width, or uniformly large orsmall features. In contrast, spot illumination problems typically causereduced space widths for small features, but may cause no noticeablewidth decrease for relatively large features. Currently, the only way tocompare corrections for a large range of different elongated spacewidths is by making multiple exposures of different test pattern cells.Separate test pattern cell exposures introduce new exposure and processvariables, which complicate the correction analysis.

Another disadvantage of two width elongated space test pattern cells isthat they do not allow evaluation of a lithography tool relative to beamshaping calibration. In particular, beam shaping is typicallyaccomplished by applying a calibrated voltage to one or moreelectrically conductive plates adjacent to the electron beam of thelithography tool to electro-statically shift the beam over an apertureto shape (and size) the emitted beam. Where a test pattern cell providesonly one or two elongated space widths, testing can only coarselyevaluate whether the lithography tool shaping deflection is accuratelycalibrated.

In view of the foregoing, there is a need in the art for improvedmethods of image quality evaluation and correction, and a test patterncell that does not suffer from the problems of the related art.

SUMMARY OF THE INVENTION

The invention addresses electron beam lithography tool image qualityevaluating and correcting and includes a test pattern with a repeatedtest pattern cell, an evaluation method and correction program product.The test pattern cell includes a set of at least three elongated spaceswith each elongated space having a different width than other elongatedspaces in the set such that evaluation of a number of space widths interms of tool image quality and calibration can be completed. Theevaluation method implements the test pattern cell in a test pattern inat least thirteen sub-field test positions across an exposure field,which provides improved focus and astigmatism corrections for thelithography tool. The program product implements the use of correctionsfrom the at least thirteen sub-field test positions to provide improvedcorrections for any selected sub-field position.

A first aspect of the invention is directed to an electron beamlithography tool test pattern cell comprising: a first set of at leastthree elongated spaces, each elongated space having a different widththan other elongated spaces in the first set.

A second aspect of the invention is directed to a method of evaluatingimage quality of an electron beam lithography tool, the methodcomprising the steps of: generating a test array of test pattern cellexposures at least thirteen sub-field test positions in an exposurefield, wherein each test pattern cell exposure within a given test arrayoccurs under a different set of lithography tool test corrections; andevaluating image quality based on the test arrays.

A third aspect of the invention is directed to a computer programproduct comprising a computer useable medium having computer readableprogram code embodied therein for correcting a lithography tool, theprogram product comprising: program code configured to determine a toolcorrection for a selected sub-field position within an exposure fieldbased on recorded test corrections for at least thirteen sub-field testpositions.

A fourth aspect of the invention is drawn to a computer-readable storagemedium having stored therein instructions for performing a method, themethod comprising the steps of: determining a lithography toolcorrection for a selected sub-field position within an exposure field ofthe lithography tool based on recorded test corrections for at leastthirteen sub-field test positions including: implementing atwo-dimensional, third-order polynomial equation for each recorded testcorrection; calculating a set of correction coefficients for eachtwo-dimensional, third-order polynomial equation; and applying the setof correction coefficients to determine the lithography tool correctionfor the selected sub-field position.

A fifth aspect of the invention is directed to a system for optimizinglithography tool image quality, the system comprising: means fordetermining a tool correction for a selected sub-field position withinan exposure field of a lithography tool based on recorded testcorrections for at least thirteen sub-field test positions, thedetermining means including: means for implementing a two-dimensional,third-order polynomial equation for each recorded test correction; meansfor calculating a set of correction coefficients for eachtwo-dimensional, third-order polynomial equation; and means for applyingthe set of correction coefficients to determine the tool correction forthe selected sub-field position.

The foregoing and other features of the invention will be apparent fromthe following more particular description of embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of this invention will be described in detail, withreference to the following figures, wherein like designations denotelike elements, and wherein:

FIG. 1 shows a prior art test pattern cell including two sets ofelongated spaces where each set has a unique width.

FIG. 2 shows a test pattern cell including a set of at least threeelongated spaces, each elongated space having a different width thanother elongated spaces in the set.

FIG. 3 shows a test array for at least thirteen sub-field test positiongenerated by repeated exposures of the test pattern cell of FIG. 2.

FIG. 4 shows a full-field test pattern including thirteen exposures ofthe test pattern cell of FIG. 2.

FIG. 5 shows a test array at one of the sub-field test positions shownin FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, a lithography tool test pattern cell 100 accordingto the invention is shown. Test pattern cell 100 includes a border 102having a first set 104 of at least three elongated spaces with eachelongated space having a different width than other elongated spaces inthe first set. In one embodiment, the elongated space widths vary from asize that is below the nominal resolution limit of a lithography tool(not shown) to be tested to a size that is several times greater thanthe nominal resolution limit of the lithography tool. In theillustrative embodiment shown, the space widths range from 100 nm to1000 nm with 100 nm increments. In particular, set 104 includes thefollowing sizes extending from an upper side of border 102 aspositioned: 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm; andthe following sizes extending from a lower side of border 102 aspositioned: 800 nm, 900 nm and 1000 nm. It should be recognized that theparticular elongated space widths chosen and the number of spaces mightbe user selected based on, for example, the types of testing, thelithography tool resolution, etc. In terms of length, in one embodiment,each elongated space may be multiple “spots” of a particular lithographytool long. A “spot” is the fundamental, unaltered exposure areaincrement of an electron beam lithography tool. For example, eachelongated space may be approximately three spots long, e.g., 6 μm for anelectron beam lithography tool having a 2 μm×2 μm full size spot.

Test pattern cell 100 also may include a second set 106 of at leastthree elongated spaces with each elongated space having a differentwidth than other elongated spaces in the second set. In the embodimentshown, second set 106 extends from left and right sides of border 102 aspositioned, i.e., oriented orthogonally compared to first set 104. Inone embodiment, the widths of first set 104 are substantially equivalentto the widths of second set 106 to aid in the analysis of imagingaberrations (e.g., astigmatism) that depend on exposed spaceorientation. Test pattern cell 100 may also include a square spot 108 ofthe maximum possible single exposure size (e.g., 2 μm square) surroundedby border 102.

To provide further testing capability, test pattern cell 100 includes atleast one shape-in-shape pattern 110. A “shape-in-shape pattern”includes a series of shaped patterns (at least two), each positionedwithin a progressively larger version of substantially the same shapepattern. It should be recognized that other shaped patterns that nestregular arrays of horizontal and vertical features are also possible. Inthe embodiment shown, shape-in-shape patterns 110 are each box-in-boxpatterns 112, 114. The elongated spaces of each box-in-box pattern 112,114 may have different sizes. For example, a first box-in-box pattern112 may include elongated spaces that are approximately 100 nm in width,and a second box-in-box pattern 114 may include elongated spaces thatare approximately 75 nm in width. It should also be recognized that thepatterns are not continuous. For example, box-in-box patterns 112, 114each include a cross blank pattern 116 and an X blank pattern 118, whichmake the shape-in-shape patterns more sensitive to resolution limitingaberrations. Test pattern cell 100 may also include an array 122 ofvaried sized square spaces 124.

By incorporating a range of elongated space widths in sets 104, 106 ofmultiple spot length and shape-in-shape patterns 110 with elongatedspace widths at or below the resolution limit of the lithography tool,certain image quality detractors may be more easily discerned. Forexample, exposure spot illumination non-uniformities are easier todistinguish from other problems when a range of elongated space widthsfrom at or below the nominal resolution limit to a space width close tothe maximum single exposure size are included. In another example, theprovision of multiple spot length spaces (e.g., 3 times the full spotdimension) ensures sensitivity to spot edge imperfections. Spot edgeimperfections may cause, for example, jagged spaces caused by shapeaperture rotation errors, spaces with bulges or gaps caused by shapeaperture image size errors, and/or horizontal and vertical space edgesrotated in different directions caused by spot off-axis astigmatism. Inanother example, a range of elongated space widths also makes itpossible to compare a range of critical dimension (CD) spacemeasurements to look for shape deflection calibration errors. Further, arange of elongated space widths enables investigation of electron beamproximity effect dose corrections and the impact of Coulomb effectelectron interactions that degrade spot focus as a function of exposedspot size (beam current). Further, shape-in-shape patterns 110 areparticularly sensitive to small variations in spot focus or astigmatism.For example, practically any rotation of shaped spot edges due tooff-axis astigmatism will be discernable by simply observingshape-in-shape patterns 110 rather than the conventional technique ofattempting to discern spot edge rotation differences between multipleimages exposed with different astigmatism corrections. Calibration ofthe electron beam lithography tool shaping deflection is also possibleusing test pattern cell 100 because the test pattern cell 100 includesthe varied width elongated spaces achieved by changing the magnitude ofthe spot-shaping offset.

Turning to FIGS. 3-5, the invention includes a method of evaluatingimage quality of a lithography tool (not shown) using a test patternincluding features such as test pattern cell 100. The method isapplicable to field position dependent image quality errors, such asspot defocus and astigmatism. The following description will detail themethod for use with test pattern cell 100. It should be recognizedhowever that the method might be used with a variety of different testpattern cells. It should also be recognized that the method does notnecessarily include every explicit step described below other than asdefined in the appended claims.

In a first step of the method, a test array 202 (FIG. 3) of test patterncell 100 exposures at at least thirteen sub-field test positionsTP1-TP13 in an exposure field 201 (FIG. 4) is generated. Evaluation ofsimilar images at multiple field positions enables correction of fieldposition dependent image quality errors, such as spot defocus andastigmatism. An exposure field 201, as shown in FIG. 4, may be made upof thousands of sub-fields. As used herein, “sub-field test position” isa sub-field for which test pattern cell 100 exposure and evaluation hasbeen completed, and “sub-field position” may be any sub-field ofexposure field 201 (tested or un-tested). A group of the at leastthirteen test arrays 202, shown in FIG. 3, constitute a composite testarray 206.

Referring to FIG. 3, in one embodiment, the test array generating stepincludes repeatedly exposing test pattern cell 100 at each sub-fieldtest position TP1-TP13 on a resist coated substrate. In one embodiment,each test pattern cell 100 exposure within a given test array 202 occursunder a different set of lithography tool test corrections. Further,each test pattern cell 100 exposure has a corresponding exposure in eachtest array that occurs under the same set of lithography tool testcorrections. In other words, each test array 202 may be considered toinclude one test pattern cell 100 exposure of a test pattern 200 asshown in FIG. 4, and each test pattern 200 includes at least thirteentest pattern cell 100 exposures that all occur under a given set oflithography tool test corrections. Between each exposure, as shown inFIG. 3, the substrate is shifted a predetermined distance (PD) such thattest pattern cell 100 exposures do not overlaop, and a test array 202 isgenerated for each test position TP1-TP13 as illustrated by compositetest array 206. “Shifting” the substrate may include movement of thelithography tool mechanical X, Y stage that holds the resist-coatedsubstrate being exposed. In addition, shifting may include movement in afirst direction or a second direction or both within a single plane. The“predetermined distance” (PD in FIG. 5) may be any distance such thatsuccessive exposures do not overlap. For a test pattern cell 100 thatis, for example, 22 μm (FIG. 2) on both sides, a predetermined distancemay be approximately 24 μm. In the embodiment shown in FIG. 3, each testarray 202 includes seven 7×7 sub-arrays 204, which results in 343exposures of test pattern cell 100 at each sub-field test positionTP1-TP13.

Images of test pattern cell 100 (and composite test array 206) areformed by exposing at least a portion of a substrate (wafer or mask)coated with an electron beam sensitive resist to an electron beam (notshown), and developing the resist coated substrate to generate testarray 202 at each sub-field test position, i.e., test array topographicfeatures. The exposed substrate is treated with a developer thatpreferentially dissolves away irradiated resist in the case of a“positive” resist process or dissolves away the non-irradiated resist inthe case of a “negative” resist process.

Generating a test array 202 of test pattern cell 100 exposures at leastthirteen sub-field test positions enables smooth tuning of the toolcorrections (i.e., spot focus and two astigmatism corrections) as afunction of position in exposure field 201, as will be described furtherbelow. The coordinates (e.g., −1044, 1044) shown in FIGS. 3 and 4indicate the location of each sub-field test position in micrometersrelative to the center (i.e., 0, 0) of exposure field 201 (FIG. 4).

Referring to FIG. 5, an illustrative test array 202 is shown. As notedabove, each exposure within a given test array occurs under a differentset of lithography tool test corrections. That is, a different set ofthe three lithography tool coil currents for focus correction, in-axisastigmatism (“stig 1 ”) correction, and off-axis astigmatism (“stig 2”)correction is used for each exposure. For example, referring tosub-array 204A of test array 202 in FIG. 5, stig 1 is varied by columnand stig 2 is varied by row. Focus correction is varied by eachsub-array 204 of 49 exposures. For example, sub-array 204A has a nofocus correction, sub-array 204B has +3 units, and sub-array 204C has −2units. “Units” may be any convenient measure for altering lithographytool settings, e.g., one or more mA for coil current. Although notnoticeable to the naked eye, variations in image quality across therange of exposure conditions (i.e., lithography tool corrections) arereadily observable when the exposed images are magnified by a factor ofapproximately 1000.

In a second step, image quality is evaluated based on test arrays 202(FIGS. 3 and 5). In one embodiment, evaluation includes determiningwhich exposure within each test array 202 at each sub-field testposition TP1-TP13 provides a highest image quality, and recording atleast one test correction (focus, stig 1, stig 2) for that exposure. Inone embodiment, this step includes selecting, at each of the sub-fieldtest positions, the best combination of focus correction, stig 1correction and stig 2 correction by evaluating the developed images witha high quality optical microscope or scanning electron microscope (SEM).When the evaluation is completed there is an optimal value for at leastone of focus correction current, stig 1 correction current and stig 2correction current for each sub-field test positions. It should berecognized that other forms of evaluation may also be carried outwithout departing from the scope of the invention.

In addition to the above evaluation, the method may also includecorrecting lithography tool image quality for any selected sub-fieldposition (tested or untested) within the exposure field. In oneembodiment, this step is achieved by determining and applying a toolcorrection(s), i.e., coil current(s), for any selected sub-fieldposition within an exposure field based on the recorded test correctionsfor the sub-field test positions, e.g., the 13 positions TP1-TP13. Thatis, at least one of a focus correction, a stig 1 correction, and a stig2 correction may be determined for any selected sub-field position. Asan alternative, a tool correction(s), i.e., coil current(s), for allsub-field positions within an exposure field may also be provided, and amechanism for recalling this data may be implemented, i.e., a lookuptable.

In order to determine/apply a tool correction for any selected sub-fieldposition, an analytical representation of the sub-field test positions'corrections are required. It should be noted that while the followingdescription is particular to implementation of 13 sub-field testpositions, the invention is applicable with more (or less) sub-fieldtest positions. In one embodiment, a two-dimensional, third-orderpolynomial is implemented for each recorded test correction, i.e., tomodel the test corrections. In this case, there is an equation for eachtest correction, i.e., focus, stig 1 and stig 2, which applies at everysub-field position, including each sub-field test position. Illustrativetwo-dimensional, third-order polynomial equations for focus correctionI_(F), stig 1 correction I_(G), and stig 2 correction I_(H) are asfollows:I _(F) =a _(F9) X ³ +a _(F8) X ² Y+a _(F7) Y ² X+a _(F6) Y ³ +a _(F5) X² +a _(F4) XY+a _(F3) Y ² +a _(F2) X+a _(F1) Y+a _(F0).   (1)I _(G) =a _(G9) X ³ +a _(G8) X ² Y+a _(G7) Y ² X+a _(G6) Y ³ +a _(G5) X² +a _(G4) XY+a _(G3) Y ² +a _(G2) X+a _(G1) Y+a _(G0).   (2)I _(H) =a _(H9) X ³ +a _(H8) X ² Y+a _(H7) Y ² X+a _(H6) Y ³ +a _(H5) X² +a _(H4) XY +a _(H3) Y ² +a _(H2) X+a _(H1) Y+a _(H0).   (3)

Each correction's two-dimensional, third-order polynomial has a set of10 polynomial correction coefficients (a₀ to a⁹) that must becalculated, i.e., a_(F0) to a_(F9) for focus correction I_(F), a_(G0) toa_(G9) for stig 1 correction I_(G,) and a_(H0) to a_(H9) for stig 2correction I_(H). The values of the 10 correction coefficients (a₀ toa₉) define the third order polynomial fit for all sub-field positionsfor a given correction, and thus yield accurate corrections for even theuntested sub-field positions. A convenient technique to handlesimultaneous linear equations is to use a matrix representation as isgiven below:

$\begin{matrix}{{\begin{matrix}I_{1} \\I_{2} \\I_{3} \\I_{4} \\I_{5} \\I_{6} \\I_{7} \\I_{8} \\I_{9} \\I_{10} \\I_{11} \\I_{12} \\I_{13}\end{matrix}} = {{\begin{matrix}X_{1}^{3} & {X_{1}^{2}Y_{1}} & {X_{1}Y_{1}^{2}} & Y_{1}^{3} & X_{1}^{2} & {X_{1}Y_{1}} & Y_{1}^{2} & X_{1} & Y_{1} & 1 \\X_{2}^{3} & {X_{2}^{2}Y_{2}} & {X_{2}Y_{2}^{2}} & Y_{2}^{3} & X_{2}^{2} & {X_{2}Y_{2}} & Y_{2}^{2} & X_{2} & Y_{2} & 1 \\X_{3}^{3} & {X_{3}^{2}Y_{3}} & {X_{3}Y_{3}^{2}} & Y_{3}^{3} & X_{3}^{2} & {X_{3}Y_{3}} & Y_{3}^{2} & X_{3} & Y_{3} & 1 \\X_{4}^{3} & {X_{4}^{2}Y_{4}} & {X_{4}Y_{4}^{2}} & Y_{4}^{3} & X_{4}^{2} & {X_{4}Y_{4}} & Y_{4}^{2} & X_{4} & Y_{4} & 1 \\X_{5}^{3} & {X_{5}^{2}Y_{5}} & {X_{5}Y_{5}^{2}} & Y_{5}^{3} & X_{5}^{2} & {X_{5}Y_{5}} & Y_{5}^{2} & X_{5} & Y_{5} & 1 \\X_{6}^{3} & {X_{6}^{2}Y_{6}} & {X_{6}Y_{6}^{2}} & Y_{6}^{3} & X_{6}^{2} & {X_{6}Y_{6}} & Y_{6}^{2} & X_{6} & Y_{6} & 1 \\X_{7}^{3} & {X_{7}^{2}Y_{7}} & {X_{7}Y_{7}^{2}} & Y_{7}^{3} & X_{7}^{2} & {X_{7}Y_{7}} & Y_{7}^{2} & X_{7} & Y_{7} & 1 \\X_{8}^{3} & {X_{8}^{2}Y_{8}} & {X_{8}Y_{8}^{2}} & Y_{8}^{3} & X_{8}^{2} & {X_{8}Y_{8}} & Y_{8}^{2} & X_{8} & Y_{8} & 1 \\X_{9}^{3} & {X_{9}^{2}Y_{9}} & {X_{9}Y_{9}^{2}} & Y_{9}^{3} & X_{9}^{2} & {X_{9}Y_{9}} & Y_{9}^{2} & X_{9} & Y_{9} & 1 \\X_{10}^{3} & {X_{10}^{2}Y_{10}} & {X_{10}Y_{10}^{2}} & Y_{10}^{3} & X_{10}^{2} & {X_{10}Y_{10}} & Y_{10}^{2} & X_{10} & Y_{10} & 1 \\X_{11}^{3} & {X_{11}^{2}Y_{11}} & {X_{11}Y_{11}^{2}} & Y_{11}^{3} & X_{11}^{2} & {X_{11}Y_{11}} & Y_{11}^{2} & X_{11} & Y_{11} & 1 \\X_{12}^{3} & {X_{12}^{2}Y_{12}} & {X_{12}Y_{12}^{2}} & Y_{12}^{3} & X_{12}^{2} & {X_{12}Y_{12}} & Y_{12}^{2} & X_{12} & Y_{11} & 1 \\X_{13}^{3} & {X_{13}^{2}Y_{13}} & {X_{13}Y_{13}^{2}} & Y_{13}^{3} & X_{13}^{2} & {X_{13}Y_{13}} & Y_{13}^{2} & X_{13} & Y_{13} & 1\end{matrix}} \times {\begin{matrix}a_{9} \\a_{8} \\a_{7} \\a_{6} \\a_{5} \\a_{4} \\a_{3} \\a_{2} \\a_{1} \\a_{0}\end{matrix}}}} & (4)\end{matrix}$

The currents (I₁ to I₁₃) are the optimized currents for a particularcorrection (i.e., coil) at each of the 13 sub-field test positionsTP1-TP13. The X and Y values represent the sub-field test positionsTP1-TP13 in microns. Correction coefficients (a₀ to a₉) for equations(1), (2) and (3) are determined from a system of linear equations asshown in matrix (4). This is an over-specified set of linear equationsso a least squares fit solution is required. Solving matrix equations ofthis form can be done using a software package such as: MATLAB®, APL® orsome math subroutines for the “C” computer language.

After the correction coefficients (a₀ to a₉) for each test correctionare calculated, they may be applied to determine a tool correction(s)for a selected sub-field position (or all sub-field positions) in theexposure field. In particular, the corresponding equations (1)-(3) maybe solved of the focus correction, stig 1 correction, and stig 2correction by inserting a selected sub-field position's X, Y coordinatesin micrometers, the same units that were used to determine thepolynomial coefficients. The result provides the tool correction for anyselected sub-field position in the exposure field. It should berecognized that while the invention has been described relative toindividual tool corrections, that a combination of corrections may beconstructed such that a collective tool correction may be implemented.

Since there is a physical limit to the maximum correction coil currentthat can be applied, the resultant focus correction, stig 1 correctionand stig 2 correction equations (1)-(3) must be verified not to specifyhigher currents than possible for any values of X and Y. Currentsexceeding the sourcing capabilities of the electronic drivers are saidto “saturate” the drivers. If saturation should occur, the coil outputshould be set to the allowable maximum value and an error message shouldbe issued. If desired, the calculated focus correction, stig 1correction and stig 2 corrections for each sub-field position in theexposure field may be placed in a lookup table. As the system steps fromsub-field to sub-field within the exposure field, the values in thelookup table are loaded to the corresponding coil drivers for focuscorrection, stig 1 correction and stig 2 correction. Since a set of toolcorrections for any X, Y sub-field position is based on a fit to all 13sub-field test positions, smooth tuning of the tool corrections (spotfocus and astigmatism corrections) across the exposure field ispossible.

In the previous discussion, it will be understood that the method stepsdiscussed are performed by a processor, such as CPU of a computersystem, executing instructions of program product stored in memory. Forexample, the method steps may be performed by program code configured todetermine a lithography tool correction for a sub-field position withinan exposure field based on a least squares third order polynomial fit atthirteen sub-field test positions for which lithography tool correctionshave been recorded. The program code may implement a two-dimensional,third-order polynomial equation for each correction (focus, stig 1, stig2) as described above. It is understood that the various devices,modules, mechanisms and systems for executing this program product maybe realized in hardware, software, or a combination of hardware andsoftware, and may be compartmentalized in various ways. They may beimplemented by any type of computer system or other apparatus adaptedfor carrying out the methods described herein. A typical combination ofhardware and software could be a general-purpose computer system with acomputer program that, when loaded and executed, controls the computersystem such that it carries out the methods described herein.Alternatively, a specific use computer, containing specialized hardwarefor carrying out one or more of the functional tasks of the inventioncould be utilized. Computer program, software program, program, programproduct, or software, in the present context mean any expression, in anylanguage, code or notation, of a set of instructions intended to cause asystem having an information processing capability to perform aparticular function either directly or after the following: (a)conversion to another language, code or notation; and/or (b)reproduction in a different material form.

While this invention has been described in conjunction with the specificembodiments outlined above, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, the embodiments of the invention as set forth aboveare intended to be illustrative, not limiting. Various changes may bemade without departing from the spirit and scope of the invention asdefined in the following claims.

1. An electron beam lithography tool test pattern cell comprising: afirst set of at least three elongated spaces, each elongated spacehaving a different width than other elongated spaces in the first set;and a second set of at least three elongated spaces, each elongatedspace having a different width than other elongated spaces in the secondset, wherein each elongated space is at least three spots long.
 2. Thetest pattern cell of claim 1, wherein the first set has substantiallyequivalent widths as the second set.
 3. The test pattern cell of claim1, wherein the first set and the second set extend from a border, andthe first set is oriented orthogonally compared to the second set. 4.The test pattern cell of claim 1, wherein each set has elongated spaceswith widths ranging from 100 nm to 1000 nm.
 5. A lithography tool testpattern containing the test pattern cell of claim 1, wherein the testpattern cell is repeated at at least thirteen sub-field test positionsof an exposure field.
 6. The test pattern cell of claim 1, farthercomprising at least one shape-in-shape pattern.
 7. The test pattern cellof claim 6, wherein the at least one shape-in-shape pattern includes afirst box-in-box pattern including elongated spaces that have a firstwidth, and a second box-in-box pattern including elongated spaces thathave a second, different width.
 8. The test pattern cell of claim 7,wherein each box-in-box pattern includes a cross blank pattern and an Xblank pattern therein.